The Milky Way

Photograph: Robert Falcetti

http://www.spacerock.nl/category/tivoli-2011/

Crater Lake, Oregon Photograph: John H. Moore

Historically, the Milky Way - seen as a faint glow across the sky - “Milk of the Heavens” - glow turns out to be countless faint stars

William & Caroline Herschel (1785) - to determine shape/size of Milky Way - counted stars in each direction

Milky Way is a flattened structure

appears as if Sun is close to center

- Limited view is due to dust - “interstellar extinction”

Harlow Shapley (1917)

- studied the distribution of globular star clusters

- they occupy a spherical region of space - but centered at a point far from Sun - towards Sagittarius

- represents the center of the MW

Now, infrared and radio observations - can map the MW

21 cm (Radio) map of the Galaxy

Galactic Center in IR, Spitzer Telescope

Solar System location

- 8.3 kpc (27,000 ly) from center

Structure of the Galaxy:

Disk: - flat distribution of stars, gas, dust - 100,000 ly across

- thin disk of young stars only 400 ly thick - thicker disk of older stars, 3000 ly thick

Spiral Arms: - located within disk - concentrations of ISM, very young stars

Halo: - spherical distribution of old faint stars - includes “dark matter”

Central Bulge: - thicker than disk, elongated shape - with a concentration of matter at center

Walter Baade (1940’s) -classified stars into two populations Population I stars:

- between 1 – 4% heavy elements - “metal-rich”

- found in the disk of galaxy

- nearly circular orbits around center

- includes bright O, B stars - concentrated in spiral arms

- less than 10 billion years old Population II stars:

- less than 0.1% heavy elements - “metal-poor”

- found in halo, globular clusters

- very elliptical, inclined orbits around center

- between 11 – 13 billion years old

Population III stars (?)

- none found, probably all have died - first generation of stars

- would have 0% heavy elements

Finding the mass of the Milky Way

- look at gravitational effect on motion of stars

- plot rotation curve

velocity (v) distance from center (r)

Solid object Mass concentrated at center

Rotation curve of MW is “flat” ?!?!

Results from other Spiral-type galaxies

Meaning of flat rotation curve:

- Much more mass beyond the outer orbits of stars

- Extra mass is completely undetectable - except for gravitational influence

- up to 10 times the mass of visible disk

“Dark Matter” Nucleus of Milky Way:

- Sagittarius A

- concentration of matter

- can observe motions of stars near center

Central mass:

- roughly 4 million Mo in a region <0.1 ly across Supermassive Black Hole

https://www.ted.com/talks/andrea_ghez_the_hunt_for_a_supermassive_black_hole

Formation of the Milky Way

Oldest stars in the galaxy (Pop. II):

- found in the halo and globular clusters - spherical distribution

- 13 billion years old

So, formation of the galaxy:

- started with the collapse of spherical “protogalactic” cloud

- almost pure H and He - tiny amounts of heavier elements

- about 13 billion years ago

- halo stars form just as collapse starts

- cloud flattens into a disk - due to rotation of cloud - keeps material spread out - can form new generations of stars

- concentration of matter at center

- forms supermassive black hole

Merger of smaller, satellite galaxies

- over time, contribute stars and gas into the Milky Way

- tidal forces stretch small galaxy - into streams of stars

- 12 separate streams of stars have been identified

- some globular clusters in halo - may be nucleus of merged galaxies

Artist’s concept of merger streams Globular cluster M54

In 3 billion years, Milky Way and Andromeda galaxies will collide

- may form a larger, round elliptical galaxy